1. Field of the Invention
The invention relates to formed articles such as manufactured articles , semi-finished articles (films, sheets, plates, etc.) made of a polymeric material and characterized by improved surface properties in the whole, and to the process for preparing them.
2. Discussion of the Background
The obtained products are broadly utilized in the packing field as they permit to obtain polymeric materials which can be printed without applicating primers and are endowed with a good adhesion along with antistatic electric characteristics as well as with good wettability, printability and clouding resistance (absence of condensate) properties.
In particular, the products obtained according to the present invention can be utilized with particular advantages in the production of sheets for greenhouses for optical components, antistatic containers for electronic components, and the like.
It is known that several technological applicative properties relating to polymers depend on their surface characteristics, more specifically on their hydrophilicity, by which other characteristics such as printability, adhesion, anti-staticity, etc. are conditioned. These are heterogeneous characteristics which raise serious technological problems as regards their simultaneous obtainment in a single finished or semi-finished article. Generally, depending on the final applicative purpose and, therefore, depending on the prevailing characteristics which are to be imparted to the article, particular technological solutions are adopted. By consequence, the resulting products hardly exhibit on the whole all the characteristics, which theoretically the article should exhibit for the particular use it is intended for, but generally they are lacking of one or the other above-cited properties such as, in particular, wettability, printability, adhesion, antistaticity, clouding resistance.
Several methods for individually improving the above-said properties are known in the art. These methods can be divided into chemical or physical surface treatments and additioning treatments.
The former can mainly consist in the chemical attack of the surface by solutions which, depending on the polymer type, can be acid, basic and/or oxidizing. However, this type of treatment involves the use of etching liquids, which are potentially dangerous and/or toxic for the work environment; furthermore it is necessary to dispose of said liquids when the use thereof is no longer required.
To the surface treatments belong also the chemical graftings, as are described for example in U.S. Pat. No. 4,728,694. In this case the drawbacks are the slow procedure and the use of chemical compounds, which are potentially dangerous, toxic for the work environment and difficult to be disposed of.
Another chemical surface treatment, which is utilized in order to obtain a better paintability, consists in applying a primer. This is a substance which gives rise to an intermediate layer between polymer and paint (or adhesive), which enhances the interaction thereof. The main drawback is represented by the necessity of applicating a very thin layer having a prefixed thickness; furthermore, the use of toxic solvents is often required.
Among the physical surface treatments, the most frequently described methods are flashing, corona discharge and the treatment with low pressure plasma. In the first two cases the drawbacks are mainly connected with the difficulty to obtain a uniform treatment on articles having a complex shape; as regards the treatments with corona discharge and with low pressure plasma, a low efficiency and a rapid decay of the effect in the long run are observed.
The other type of procedure for obtaining the hydrophilization of the polymeric surfaces consists in adding the polymer with proper polar chemical compounds tending to migrate to the surface. The drawbacks reside in the difficulty to dose the additive. A wrong dose or a non-homogeneous distribution can result in a lack of efficiency of the additive or in a loss of efficiency in the long run. Furthermore, toxicity problems can arise in the work environment, which are connected with the generation of fumes during the extrusion and molding of the added polymer.
Moreover it must be pointed out that the above-described methods are typically partial, i.e. they are capable of solving only some of the problems depending on the low surface tension (wettability) of the polymers Typically, the flashing, corona discharge and plasma treatments can be utilized to improve the wettability, printability and adhesion, but they are substantially ineffective as regards an improvement in the antistaticity and clouding resistance conversely, the adding can be utilized to improve the antistaticity and clouding resistance, but it is typically ineffective in improving the adhesion and printability.
In conclusion, the prior art--as far as the Applicant knows at present--seems not to be capable of providing a method for the production of articles endowed with a sufficiently wide range of satisfactory characteristics and therefore suited to be directly utilized in a broad field of uses.